Islay, with your academic hat on then , do you agree that :
1. Society has a duty of care for every citizen whilst they are a child - that duty is absolute and nothing the child does (however repugnant) can allow Society to abdicate that responsibility - Society may choose to punish/control the child to prevent abnormal and/or criminal behaviour, but cannot just wash its hands of someone.
2. Once a citizen is an adult the contract becomes two-way - Society has a duty of care to the citizen , but the citizen has a duty of care to Society (which, let's remember, is composed of individuals as well as being an amorphous entity).
If the above statements are accepted as true, then the argument is reduced quite simply to an evaluation of whether the citizen has abdicated his responsibility to Society?
I think Venables has undoubtedly done that, so what should Society do to protect itself (given that it no longer has any duty of care for the miscreant)?